The observed correlation can be successfully explained on the basis of the Lense-Thirring (LT) precession model [1]}, [2]} and the truncated disc model [3]}, [4]}, [5]}. When the mass accretion rate shows rapid growth, the truncated disk radius slowly starts moving towards the compact object, hence leading to an increase of the LT precession frequency. The QPO frequency and its evolution are governed by the size of as well as the fluctuations in the truncation radius (increase in QPO frequency represents a decrease of the truncation radius)[6]}. The spectral changes during the source transition can also be explained in this framework. When the mass accretion rate increases, the outer disk gradually starts moving towards the compact object. The disk component becomes stronger and causes greater cooling of the hot inner flow by the cool photons from the disk, resulting in a soft spectrum [7]}, [8]}.
